1. Field of the Invention
The present invention generally relates to a novel lighting circuit for a high-pressure discharge lamp for vehicles. More particularly, this invention is directed to a novel lighting circuit for a high-pressure discharge lamp for vehicles, which, in addition to a DC (direct current) voltage booster circuit, has a feedback circuit that detects the output voltage and output current of the DC voltage booster circuit and sends a control signal corresponding to the detection result to the booster circuit, whereby, upon lighting of the discharge lamp, the flux of light from the lamp can reach a rated level in a considerably short period of time. This invention aims at significantly enhancing the practical use of metal halide lamps which are now attracting attention as a light source for vehicular headlamps.
2. Description of the Related Art
With regard to automobiles, there is a growing demand to improve the safety in driving at night and the aerodynamic characteristics of the body of a car so as to reduce fuel consumption. This demand also applies to the headlamps of an automobile; the safety improvement requires better visual confirmation of the headlamps while the improvement of the aerodynamic characteristics requires the headlamp being formed in a slanted shape or made more compact and flatter.
With respect to the reduction in fuel consumption, metal halide lamps have been receiving greater attention due to their having much higher light source efficiency and longer service life than halogen lamps.
A metal halide lamp has an igniting gas (argon, etc.), mercury and metal iodide filled in a glass bulb. When a high voltage is applied to a discharge electrode of this lamp, a mercury arc is generated after a discharge of the igniting gas is started, thus generating heat. The generated heat vaporizes the metal iodide, dissociating in the mercury arc, thus causing irradiation of a strong flux of light which has a specific spectrum of the metal.
A conventional lighting circuit for a high-pressure discharge lamp including such a metal halide lamp is disclosed in, for example, Unexamined Japanese Patent Publication No. 62-259391.
In order to turn on a high-pressure discharge lamp using a DC power supply, the disclosed conventional circuit comprises a DC power supply, an up-converter connected to the power supply for voltage boosting, a sinusoidal converter connected to the up-converter for converting the DC voltage from the up-converter into a sinusoidal AC (alternate current) voltage, and a starter circuit. Supplying a sinusoidal AC voltage at the time of lighting the discharge lamp eliminates unstable operation of the discharge lamp which would otherwise likely be caused by a rectangular AC voltage. Further, designing the up-converter to serve as a controllable DC voltage converter assures adjustment of the output of the lighting circuit.
Although the above-described lighting circuit can permit a high-pressure discharge lamp to be lit by a DC current, it takes time to attain a specified brightness after the discharge lamp is initially lit (starting time) or when the discharge lamp is turned on again after being turned off temporarily (restarting time). This shortcoming is fatal to a headlamp.
The source of this problem is as follows. When discharge starts with the glass bulb of the discharge lamp being cool (this event will be hereinafter referred to as "cold starting"), it takes time for the metal iodide in the glass bulb to be vaporized, and, when the discharge lamp is lit again after temporarily being turned off, the pressure in the glass bulb becomes significantly high upon elapse of a certain time, thus increasing the discharge starting voltage. There are also nonnegligible external factors such as ambient temperature.